Wastewater, as well as un-altered waters characterized by high salt concentrations (>1,000 mg/L), are generally referred to as saline water. Highly saline wastewaters are produced in an array of industrial applications including during the exploration for and production of oil and natural gas resources from subsurface formations. For example, the management of produced waters from oil and gas production is receiving growing scrutiny as the volumes that are now being generated in the United States alone are approaching 20 to 30 billion barrels per year. Produced waters can have total dissolved solids concentrations ranging from thousands to several hundred thousand mg/L. In addition, numerous natural sources of saline or brine water (e.g., brackish groundwater) exist that are not suitable for use without treatment.
The largest use of freshwater across the world is for the irrigation of crops for human and livestock food production. The continued growth of the world's population in conjunction with finite freshwater resources has increased the interest in reuse of ‘impaired/waste’ waters or waters naturally enriched with salts for irrigation of crops. The paradigm has shifted from considering these effluent streams as ‘waste’ to now seeing them as a valuable resource. Realizing the possibility of reusing brine/saline waters for some beneficial purpose requires that existing technical and economic challenges associated with desalination processes be overcome. For irrigation these challenges largely focus on the technical complexity, chemical inputs, and energy consumption required for desalination.
Aqueous solutions containing dissolved salts (e.g., brine/saline streams) are usually purified (desalinated) by ion exchange, pressure driven membrane processes (e.g., nano-filtration, reverse osmosis), mechanical distillation, and crystallizers. While effective, these technologies are highly energy intensive and require extensive infrastructure, and thus are costly. Accordingly, there has recently been significant interest in non-pressure driven membrane processes (forward osmosis, membrane distillation), which typically have lower energy requirements.
Forward osmosis is the transport of a solvent (e.g., water) across a water permeable membrane from a region of lower osmotic potential to a region of higher osmotic potential. During this process solutes (e.g., salts) are rejected by the membrane. Stated otherwise a feedwater solution (e.g., waste water) on one side of the membrane has a lower osmotic potential than the osmotic potential of a draw solution (e.g., osmotic agent) on an opposing side of the membrane. Water passes across the membrane to equalize the osmotic potential thereby concentrating the feedwater solution and diluting the draw solution. While effectively removing water from a wastewater stream, this process requires removal of the water from the draw solution prior to being put to a beneficial use. That is, producing desalinated water using a forward osmosis process requires two steps, forward osmosis membrane separation and separation of water from a diluted draw solution/osmotic agent. In conventional industrial settings, the first step is energy efficient, the second step is not.
Recovery or separation of water from the diluted draw solution typically requires considerable energy input. For instance, water is sometimes separated from draw solutions using reverse osmosis. In other instances, diluted draw solutions are distilled (e.g., boiled off) to remove the draw solution form the water or vice versa. The energy expended to separate the water from the draw solution often negates the benefits of the forward osmosis process. Accordingly, forward osmosis has not found widespread acceptance for wastewater recovery.